1. Field of the Invention
The present invention relates to gas turbine engines and, more specifically, to combustors for gas turbine engines.
2. Description of the Related Art
Nitrous oxides, hereinafter NO.sub.x, are formed during combustion of fuel with air. Recent investigations and experimentation lead to the concusion that all NO.sub.x formation is "prompt NO.sub.x ", i.e., NO.sub.x formed during a non-equilibrium combustion process occurring a very short period of time, a few milliseconds, after initiation of the combustion process. It has only recently been postulated that such a non-equilibrium condition creates a severe temperature spike which rapidly decays to the equilibrium temperature, and that substantially all NO.sub.x is formed during these high peak temperatures. This observation has lead to the conclusion that formation of NO.sub.x is independent of residence time within a combustion chamber but is exponentially related to the temperature at which combustion occurs. Such a conclusion is in contradiction to conventional thinking which relates NO.sub.x formation to residence time.
FIG. 1 shows the experimental relationship between NO.sub.x formation and flame temperature. In this figure, the temperature is the equilibrium flame temperature and the amount of NO.sub.x is the sum of all NO.sub.x formed as the temperature drops from its initial high value to the equilibrium value. The amount of NO.sub.x is shown in FIG. 1 as a log value. Hence, while the curve of FIG. 1 is substantially straight, it in fact reflects the exponential relationship to flame temperature.
Because combustion systems using air as the oxygen source always contain mostly nitrogen, and because the relaxation time from the non-equilibrium to equilibrium condition depends solely on the molecules involved in the combustion process, the curve of FIG. 1 is valid for any air-breathing combustion system. Furthermore, the NO.sub.x formation rate at the equilibrium temperature conditions has been shown to be so low that it does not measurably affect the amount of NO.sub.x formed in normal combustion systems where the gas is at the equilibrium temperature for times of a few seconds or less.
Thus, it is an object of the present invention to provide a premixed, convection cooled, low NO.sub.x emission combustor having structural features which take advantage of the conclusion that substantially all NO.sub.x formation is "prompt NO.sub.x " related only to the temperature at which combustion occurs and not related to the residence time within the combustion chamber.
It is a further object of the present invention to provide a combustor for a gas turbine engine having improved abilities to vaporize and mix the fuel and air prior to being burned in the combustion chamber.
It is still a further object of the present invention to provide a combustor configuration for a gas turbine engine having a convection cooling air flow passage surrounding the hot wall of the combustor which is substantially free of obstructions to thereby enhance the effectiveness of the cooling air flow through the passages. Such a construction also simplifies the mechanical design of the combustor, reduces manufacturing costs, and simplifies inspection procedures.
It is still a further object of the present invention to provide a combustor configuration which requires fewer fuel injection nozzle than present designs.
It is also an object of the present invention to provide a combustor configuration having a combustion chamber which is separated into primary and secondary combustion zones wherein burning of fuel and air in the primary combustion zone occurs at a reduced flame temperature thereby reducing formation of NO.sub.x.
It is still a further object of the present invention to provide a combustor configuration adapted for convection cooling of the combustor wall wherein all the cooling air is used in the combustion process for either combustion with the fuel or for dilution of the products of combustion to reduce the temperature of the gas entering the turbine.
It is still a further object of the present invention to provide a combustor configuration which reduces the amounts of unburned hydrocarbons and carbon mono-oxide.